1. Technical Field of the Invention
The present invention relates to a noise filter, and, more particularly to a noise filter for Bayer pattern image data. The invention also relates to a method of filtering noise in Bayer pattern image data.
2. Description of Related Art
Digital cameras generate a datafile that represents an image acquired by the camera. Generally, the camera acquires the information from the light/color sensors in the camera in a CFA (camera filter array) format. A popular format for the CFA is a Bayer mosaic pattern layout, shown in FIG. 1. In a Bayer pattern, each pixel contains information that is relative to only one color component, for instance, Red, Green or Blue. Typically, the Bayer pattern includes a green pixel in every other space, and, in each row, either a blue or a red pixel occupies the remaining spaces. For instance, as seen in FIG. 1, row one alternates between green and red pixels, and row two alternates between green and blue pixels. The end result is a mosaic made of red, green and blue points, where there are twice as many green points as red or blue. This can accurately represent an image because the human eye is more sensitive to the green data than either the red or blue.
A typical camera includes a charge coupled device (CCDs) or CMOS image sensor that is sensitive to light. These image sensors are sensitive only to the intensity of light falling on it and not to the light's frequency. Thus, an image sensor of this kind has no capability to differentiate the color of light falling on it.
To obtain a color image from a typical camera sensor, a color filter is placed over the sensitive elements of the sensor. The filter can be patterned to be like the Bayer pattern discussed above. Then, the individual sensors are only receptive to a particular color of light, red, blue or green. The final color picture is obtained by using a color interpolation algorithm that joins together the information provided by the differently colored adjacent pixels.
The images produced by digital still cameras, especially ones produced by CMOS technology, suffer from noise that is inherently present in the sensor when the image is captured. Thus, some modification of the datafile produced by the image sensor is needed. Oftentimes this modification comes as a filter or algorithm run on the image sensor data (the Bayer Pattern) to create a more realistic image. Processes are performed on it in an imaging process chain, for example, white balance, gamma correction, etc., as shown in FIG. 2. Finally, the corrected image is compressed and stored in memory as a color image.
One problem with current image processing in current digital camera systems is that noise artifacts are difficult to differentiate from detail in the image itself. If noise artifacts are thought to be detail and left unfiltered, or details are thought to be artifacts and are filtered, image degradation occurs.
Another problem exists in that current filtering methods operate on the image data as a whole and do not differentiate between the individual color channels making up the image.
The technical problem solved by this invention is one of how to best filter a complex image file for noise, while at the same time preserving detail in the image.